Generally, crystalline defects in substrates for microelectronic devices are highly undesirable because of their negative impact on functionality and reliability of integrated circuits formed on such substrates. For the delineation of crystalline defects and accordingly for the characterization of the quality of such substrates, structural etching solutions are widely employed. With respect to crystalline defects, the etch rate of structural etching solutions differ, leading to the production of hillocks or etch pits, which can then be visually observed.
Germanium (Ge) has increasingly been studied in relation to its use as substrate for microelectronic devices, due to the intrinsic advantages of this element, such as high intrinsic electron and hole mobility. In order to fabricate high performance devices, it is however essential to understand germanium surface chemistry and to find effective ways to clean and analyze its surface.
Surface treatment methods in particular have so far been developed for silicon surfaces, although approaches also have been made in order to treat germanium surfaces in order to characterize and visualize defects as contained.
One well known structural etching solution, already widely used for silicon substrates, is a dilute aqueous solution of an alkali dichromate, as for example described by F. Secco d'Aragona in Journal of the Electrochemical Society 119, No. 7, 948-951 (1972). This etch already has been used on germanium substrates with highly satisfactory results, such as good etch rate and reliable defect identification. However, the use of dichromate-containing solutions is considered as being unfavorable due to the toxic nature of the dichromate compounds, which are known to cause severe damages to human health. Accordingly, differing structural etching solutions have been developed for treating germanium surfaces, such as the etchant disclosed in Applied Physics Letters, Vol. 75, No. 19, pages 2909-2911, comprising acetic acid, nitric add, hydrofluoric acid, and iodine. A similar etchant is also disclosed in Applied Physics Letters, Vol. 85, No. 11, pages 1928-1930.
Other etchants employed for germanium surfaces are solutions comprising hydrofluoric acid, hydrogen peroxide, and acetic acid in a 1:1:1 ratio as well as the simple use of hydrochloric acid and hydrofluoric acid, respectively, for removing oxide coatings on germanium surfaces, as disclosed in Applied Physics Letters, Vol. 60, No. 7, pages 844-846, and Applied Physics Letters 88, the article extending from page 021903-1 to 021903-3.
Finally, a two-step treatment regiment has been disclosed in ECS Transactions, 6(4), 263-269 (2007), comprising a dip of a germanium surface in a mixture of hydrofluoric acid, nitric acid, and acetic acid, followed by a short dip into hydrogen peroxide.
While most of the etchant compositions disclosed above show a suitable ability for treating germanium surfaces, it is nevertheless still required to improve the quality of the surface treatment, in order to enable a high quality characterization of the treated surface and also enabling the treatment of rather thin germanium substrates, such as GeOI substrates, without suffering from the drawback of undesired film delamination due to undesired etching progress.